What is the best and most cost affordable solution for storing user uploaded videos and pictures?

Question

I'm in the process of building an iOS application that will allow users to upload short videos (low res. and < 5 seconds) as well as full-size images as taken by their iOS device. Not being certain of how successful this endeavor will be I'm looking for something solid, cost affordable and can scale as (if) needed.

Here are my requirements in a nutshell:

1. Users will be able to upload low res. vides < 5 sec. in length or full-sized images captured from their iOS device.

2. I need the ability to monitor the amount of storage used on a user-by-user basis for billing and even restricting further uploads if, for example, they have reached a certain threshold

3. I need something robust but cost effective (I'm a one man operation at this point). Something that can grow and scale with me would be ideal.

Any help in reaching this goal is definitely appreciated.

Thanks -wg